What is in MPI?

A lot! MPI is a very comprehensive library and MPI-2 has about all of it including some elements of fault-tolerance, process migration, etc.

The most fundamental functions in MPI, MPI_Send  and MPI_Recv , provide not only for buffered sends and receives, but also for typing  of transferred data. They also support the concept  of a communicator. A communicator is a group of processes within which the ranking , i.e., process differentiation, and communication take place. A single program may comprise several overlaping or completely separate communicators. This concept is crucial to implementations of parallel code libraries. Otherwise, there would always be a risk of library communications interfering with other communications within the parallel program.

MPI provides a very rich set of collective communication functions, virtual topologies, hooks for debugging and profiling, blocking and non-blocking sends and receives, and support for heterogeneous networks. MPI-2  adds numerous clarifications, specification of portable MPI process startup (previously every MPI implementation would use different ways of starting MPI programs), new data type manipulation functions and new predefined types, support for dynamic process creation and management, support for one-sided communications, i.e., for the ability to write data directly into other processes' memories, portable high-performance IO in the form of MPI-IO , and support for  C++ and  Fortran 90.

MPI is obviously a very large library. The listing below shows all MPI-1 functions. There are 128 of them. MPI-2 adds at least this many if not more.

MPI_ABORT	MPI_ADDRESS	MPI_ALLGATHER	MPI_ALLGATHERV
MPI_ALLREDUCE	MPI_ALLTOALL	MPI_ALLTOALLV	MPI_ATTR_DELETE
MPI_ATTR_GET	MPI_ATTR_PUT	MPI_BARRIER	MPI_BCAST
MPI_BSEND	MPI_BSEND_INIT	MPI_BUFFER_ATTACH	MPI_BUFFER_DETACH
MPI_CANCEL	MPI_CARTDIM_GET	MPI_CART_COORDS	MPI_CART_CREATE
MPI_CART_GET	MPI_CART_MAP	MPI_CART_RANK	MPI_CART_SHIFT
MPI_CART_SUB	MPI_COMM_COMPARE	MPI_COMM_CREATE	MPI_COMM_DUP
MPI_COMM_FREE	MPI_COMM_GROUP	MPI_COMM_RANK	MPI_COMM_REMOTE_GROUP
MPI_COMM_REMOTE_SIZE	MPI_COMM_SIZE	MPI_COMM_SPLIT	MPI_COMM_TEST_INTER
MPI_DIMS_CREATE	MPI_ERRHANDLER_CREATE	MPI_ERRHANDLER_FREE	MPI_ERRHANDLER_GET
MPI_ERRHANDLER_SET	MPI_ERROR_CLASS	MPI_ERROR_STRING	MPI_FINALIZE
MPI_GATHER	MPI_GATHERV	MPI_GET_COUNT	MPI_GET_ELEMENTS
MPI_GET_PROCESSOR_NAME	MPI_GRAPHDIMS_GET	MPI_GRAPH_CREATE	MPI_GRAPH_GET
MPI_GRAPH_MAP	MPI_GRAPH_NEIGHBORS	MPI_GRAPH_NEIGHBORS_COUNT	MPI_GROUP_COMPARE
MPI_GROUP_DIFFERENCE	MPI_GROUP_EXCL	MPI_GROUP_FREE	MPI_GROUP_INCL
MPI_GROUP_INTERSECTION	MPI_GROUP_RANGE_EXCL	MPI_GROUP_RANGE_INCL	MPI_GROUP_RANK
MPI_GROUP_SIZE	MPI_GROUP_TRANSLATE_RANKS	MPI_GROUP_UNION	MPI_IBSEND
MPI_INIT	MPI_INITIALIZED	MPI_INTERCOMM_CREATE	MPI_INTERCOMM_MERGE
MPI_IPROBE	MPI_IRECV	MPI_IRSEND	MPI_ISEND
MPI_ISSEND	MPI_KEYVAL_CREATE	MPI_KEYVAL_FREE	MPI_OP_CREATE
MPI_OP_FREE	MPI_PACK	MPI_PACK_SIZE	MPI_PCONTROL
MPI_PROBE	MPI_RECV	MPI_RECV_INIT	MPI_REDUCE
MPI_REDUCE_SCATTER	MPI_REQUEST_FREE	MPI_RSEND	MPI_RSEND_INIT
MPI_SCAN	MPI_SCATTER	MPI_SCATTERV	MPI_SEND
MPI_SENDRECV	MPI_SENDRECV_REPLACE	MPI_SEND_INIT	MPI_SSEND
MPI_SSEND_INIT	MPI_START	MPI_STARTALL	MPI_TEST
MPI_TESTALL	MPI_TESTANY	MPI_TESTSOME	MPI_TEST_CANCELLED
MPI_TOPO_TEST	MPI_TYPE_COMMIT	MPI_TYPE_CONTIGUOUS	MPI_TYPE_EXTENT
MPI_TYPE_FREE	MPI_TYPE_HINDEXED	MPI_TYPE_HVECTOR	MPI_TYPE_INDEXED
MPI_TYPE_LB	MPI_TYPE_SIZE	MPI_TYPE_STRUCT	MPI_TYPE_UB
MPI_TYPE_VECTOR	MPI_UNPACK	MPI_WAIT	MPI_WAITALL
MPI_WAITANY	MPI_WAITSOME	MPI_WTICK	MPI_WTIME

But you don't have to panic. In great many cases you can do all you need with just 6 fundamental MPI functions:

MPI_Init

Initialize  MPI processes;

MPI_Comm_size

Find  out about the number of the processes in the MPI communicator;

MPI_Comm_rank

What  is my rank number within the pool of MPI communicator processes?

MPI_Send

Send a message.

MPI_Recv

Receive a message.

MPI_Finalize

Close  down MPI processes and prepare for exit.

All the other functions, 128 minus 6, are auxiliary. They make programmer's life much, much easier by encapsulating frequently used parallel programming procedures in convenience functions.